Organic Techniques - Purity
Sample Preparation: Solids must be thoroughly dried and crushed into a fine powder using a spatula. This powder is packed tightly into a glass capillary tube to a depth of approximately to ensure efficient thermal contact.
Apparatus Selection: Common tools include the Thiele tube or an electronic melt station. In a Thiele tube, an oil bath is heated at the side arm; this creates convection currents that ensure the temperature of the oil surrounding the sample is uniform and accurately reflected by the thermometer.
Heating Protocol: The procedure involves two stages: a rapid initial heating to find the approximate melting point, followed by a precise run where the temperature is raised very slowly (about per minute) as the expected melting point is approached.
| Property | Pure Substance | Impure Substance |
|---|---|---|
| Melting Point | High and Sharp | Lowered and Broadened |
| Boiling Point | Accurate to Literature | Elevated and Variable |
| Range Width | Typically | Often |
Recording Ranges: Always record melting points as a range (e.g., ). Examiners look for the start temperature (first liquid droplet) and the end temperature (complete liquefaction).
Verification Steps: If a measured melting point is significantly lower than the literature value, the most likely conclusion is that the sample is impure or contains residual solvent. If it is higher, the sample may be a different compound entirely or the thermometer may be uncalibrated.
Sanity Checks: Ensure the oil used in a Thiele tube has a boiling point significantly higher than the melting point of the sample to avoid the oil boiling during the experiment.
Thermal Lag: Heating the apparatus too quickly causes the thermometer to register a higher temperature than the sample has actually reached. This leads to inaccurate readings and an artificially broad range.
The 'Wet Sample' Error: Residual solvent from the purification process acts as a powerful impurity. Students often mistake a 'wet' sample for a pure one with a low melting point; thorough drying is essential before testing.
Loose Packing: If the powder in the capillary tube is not packed tightly, air pockets act as insulators, leading to uneven heating and inconsistent melting behavior.